The present invention relates in general to electronic circuits and components therefore, and is particularly directed to a new and improved voltage-controlled, modified Brokaw cell-based current generator, which is operative to generate an output current that exhibits a linear temperature coefficient.
A variety of electronic circuit applications employ one or more voltage and/or current reference stages to generate precision voltages/currents for application to one or more loads. In order to accommodate parameter (e.g., temperature) variations in the environment in which the circuit is employed, it is often desirable that the reference circuit""s output conform with a prescribed behavior. In the case of a voltage reference, for example, it is common practice to employ a precision voltage reference element, such as a xe2x80x98Brokawxe2x80x99 bandgap voltage reference circuit, from which an output or reference voltage having a relatively flat temperature coefficient may be derived.
A reduced complexity circuit diagram of such a Brokaw bandgap voltage reference circuit is shown in FIG. 1 as comprising a pair of bipolar NPN transistors Q1 and QN, having their bases connected in common and to a bandgap voltage (VBG) output node 11. In a typical integrated circuit layout, transistors QN and Q1 are located adjacent to one another and differ only in terms of the geometries by their respective emitter areas by a ratio of N:1. Alternatively, transistor QN may correspond to a plurality of N transistors coupled (or xe2x80x98lumpedxe2x80x99) in parallel. The collectors of transistors QN and Q1 are coupled to respective ports 21 and 22 of a current mirror 20. The current mirror and amplifier makes an equal current flowing though the collector of QN and Q1. Transistor Q1 has its base-emitter junction voltage VbeQ1 derived from the series connection of the base-emitter junction of transistor QN and resistor R1, and its emitter Q1e coupled to the current summation node 12. Current summation node 12 is coupled through a resistor R2 to ground.
In the Brokaw cell voltage reference circuit of FIG. 1, the voltage on the R1 is equal to the VBE difference of the transistor Q1 and QN, which is proportional to absolute temperature (or PTAT) and is definable as (kT/q)lnN, where k is Boltzman""s constant, q is the electron charge, T is temperature (in degrees Kelvin), N is the ratio of the emitter areas of transistors QN/Q1. The PTAT current 11 supplied through the resistor R2 produces a PTAT voltage thereacross, which is (2*R2/R1)*(kT/q)*lnN, where R1 and R2 are the resistance of resistor R1 and R2 respectively. This PTAT voltage VPTAT is summed with the VBE voltage across transistor Q1 (which is complementary to absolute temperature or CTAT), to derive an output voltage reference VBG at output terminal 11. As shown in FIG. 2, the output reference voltage VBG produced by the Brokaw bandgap reference circuit of FIG. 1 has a first-order compensated temperature coefficient, which typically varies in a xe2x80x98squeezedxe2x80x99, generally parabolic manner between 20 to 100 ppm/xc2x0 C.
In addition to the need for circuits that exhibit an essentially flat voltage vs. temperature characteristic, such as the Brokaw voltage reference described above, there are a number of applications where it is desired that an output current vary in a prescribed manner with change in temperature. For example, in the case of a battery charger, it may be desirable to generate an output current that exhibits a well defined linear slope over a given temperature range for the thermal fold back.
In accordance with the invention, this objective is realized by employing the temperature dependency functionality exhibited within the circuitry used to generate Brokaw voltage reference, so as to realize a modified Brokaw cell-based circuit that produces an output current whose temperature coefficient varies linearly with temperature. In the modified Brokaw cell based circuit of the invention, Q1 and QN is exchangeable. The collector-emitter current flow path the transistor QN of the Brokaw circuit of FIG. 1, rather than being connected to the current mirror port, is connected to a diode connection in series with the collector-emitter current flow path of a control transistor. The base of the input transistor is coupled to receive an input or xe2x80x98referencexe2x80x99 (control) voltage VREF, whose value defines a limited linear range of variation of output current with temperature. The collector of the output transistor Q1 is coupled to an input port of a current mirror, which mirrors the collector current from output transistor at an output port thereof.
Unlike the conventional Brokaw circuit of FIG. 1, whose output is xe2x80x98voltagexe2x80x99 and whose input is a xe2x80x98currentxe2x80x99 supplied by a current mirror connected to two the legs of the voltage reference circuit, the output of the modified Brokaw circuit of the invention is a xe2x80x98currentxe2x80x99 that varies linearly with temperature, and its input is a control xe2x80x98voltagexe2x80x99 applied to the base of its control transistor. For a given reference voltage applied to its base, the control transistor will produce a prescribed (PTAT) output current, which is applied to the collector-emitter current flow path of the diode-connected transistor QN and thereby to the series connected resistors R1 and R2. The collector current of the output transistor Q1 is defined in accordance with the sum of the voltage drop VR1 across the resistor R1 and the base emitter voltage VbeQN of transistor QN. Since the voltage variation across the resistor R1 is PTAT (and is dominant) and that of the VbeQN of transistor QN is CTAT, the resultant Vbe of the output transistor is the sum of a dominant PTAT component and a CTAT component, and has a linear temperature coefficient.
Operational conditions, such as slope and DC offset, of the current generator of the invention may be selectively defined in accordance one or more parameters or relationships among parameters of the circuit. For example, the slope of the linear variation of the output current with temperature may be varied by varying the ratio of the emitter areas of transistors Q1 and QN and/or by the ratio of the values of resistors R1/R2. For a given temperature, the output current may be varied by changing the magnitude of the control voltage applied to the base of the control transistor.
The ability of the invention to produce an output current that exhibits a very linear variation with temperature makes its readily adaptable to a variety of applications requiring customized temperature-based current behavior characteristics. For example, multiple current generators of the present invention having different parameter settings may be combined to produce a composite piecewise linear variation with temperature. As a non-limiting example, a first output current whose variation with temperature has a zero slope may be combined with a second output current having a substantial non-zero slope over its linear temperature variation, to produce a piecewise flat then inclining or declining variation with temperature current behavior.